The Miniature Neutron Source Reactor (MNSR) is manufactured by the Chinese Institute of Atomic Energy. It is a low power, tank-in-pool type research reactor and similar to the canadian SLOWPOKE reactor. The reactor employs highly enriched uranium as fuel (90.2%), light water as moderator and coolant, and metal beryllium as reflector. In this thesis, the Isfahan Miniature Neutron Source Reactor (MNSR) was simulated using the WIMS code and the calculations of fuel burn-up was performed after 7 years of the reactor operation time (reactor revival time by adding the top beryllium shim plate) the results of which were consistent with the expected results from other methods. Also the MNSR reactor was simulated using the MCNP code that works on the basis of the Monte Carlo method and after confidence from the accuracy of the simulated program, the results obtain from the WIMS code were incorporated in the input of the MCNP program. the necessity of adding the 1.5 millimeter of top beryllium shim plate after 7 years of the reactor operation time was studied and then by adding the 1.5 millimeter beryllium layer at the top of the reactor core in the input of the MCNP program, reactivity variations was studied in new status the results of which were consistent with experimental results. Furthermore by adding beryllium layer with various thicknesses at the top of the reactor core in the input program of the MCNP program by using reactor fresh fuel and once more after 14 years of the reactor operation time, variations of reactivity was studied and worth of reactivity was obtained for these layer the results of which were consistent with available experimental results very well. Also reactor status at present time (after 14 years of the reactor operation time and after 7 years of adding the first beryllium layer at the top of the reactor core) was simulated using the WIMS and MCNP codes and reactor reactivity at this time was obtained and reactor’s need for adding new beryllium layer at the top of the core was studied. Furthermore reactor reactivity was measured by experimental method the results of which were consistent with those results that were obtained from the WIMS and MCNP codes and we concluded that at present time there is no need for adding any further beryllium shim to the top of the MNSR reactor core. At the end worth of reactivity for the annulus beryllium and bottom beryllium were calculated on the basis of the worth of one fuel element using the MCNP code. From the results obtained in the last section of these calculations, one can predict the necessary beryllium thickness needed to rich a desired reactivity in the MNSR reactor. Key Words MNSR reactor, Reactivity, Burnup, Beryllium, WIMSD code, MCNP code.